The present invention concerns an unbalance measurement device for determining the unbalance of a rotor according to position and magnitude, and more particularly to an unbalance measurement device for a balancing machine having a drive motor, transducers, a spindle supporting the rotor to be balanced, and an increment disc fixed to the spindle, the measurement device including analog filter circuitry for analog signals to reduce extraneous vibrations by means of controlling its filter mid-frequency by a high control frequency.
In all balancing machines it is necessary to eliminate extraneous vibrations generated by the bearings of the body to be balanced, the drive of the body to be balanced, or by air vibrations of the body itself, all of which influence the indication for position and/or amount of unbalance. For this purpose, the use of electrical filters has gained acceptance besides multiplying measuring systems, mainly with simpler machines, e.g. also for wheel balancing machines. For this purpose, band filters, high-pass filters, or low-pass filters are used.
However, all circuits of this nature have the disadvantage that already at minor deviations in the balancing speed from the predetermined mid-frequency of the filter device, significant phase errors of the transferred measurement signals will occur. Since unbalance measurement devices on balancing machines should indicate not only the magnitude of the unbalance but also the position of the unbalance on the body to be balanced in one or several planes, each error in the phase transmission has the effect of an error to the correction of the unbalance on the body to be balanced.
Electrical filters can be designed with varying filter selectivity Q, whereby unfortunately those filters which have the best selection effect also have the greatest change in phase response. If an electrical filter of high selectivity Q is used in unbalance measurement devices for balancing machines, care must be taken that the drive motor very precisely maintains the predetermined test speed of the rotational body. Speed deviations will thus cause major phase errors, particularly with electrical filters of high filter selectivity. Accordingly, there is currently an effort to reach a compromise between filter selectivity and phase error in order to be able to permit insignificant speed fluctuations within the range of the measurement speed without obtaining too great a phase error, which would cause an imperfect correction of the unbalance.
It would be obvious to apply "phase locked loops" (PLL) to the balancing speed for purposes of auxiliary control of the filters. However, since the phase itself is utilized as a control means in such control circuits, "PLL filter followers" will show phase errors. The magnitude of these errors depends on the deviation of the mid-frequency of the filter from the signal frequency. Signal frequency is understood to mean, in this case, the rotation frequency which usually changes from revolution to revolution of the body to be balanced. Such a circuit arrangement will by necessity generate a phase error, since the control circuit needs a value for the phase error in order to control the mid-frequency of the filter whereby the phase error does not reach a value of zero. Thus, this device is not appropriate for representing a value of an angle of unbalance which is free of phase errors.
UK Patent Application No. 20 41 538 also discloses a device for the subsequent compensation of these phase errors by means of complicated circuitry. According to this publication, the phase error is artifically determined by means of a special device and then added, by means of a microprocessor computer circuit as correction to the measured angle value, which is per se incorrect.